The term “carbon nanostructures” includes structures such as fullerenes, carbon nanotubes (CNTs), carbon nanofibres, carbon nanoparticles, carbon nanoplates, and graphene. Graphene, in particular, possesses many extraordinary properties such as high ballistic electron mobility, high thermal conductivity, high Young's modulus, high fracture strength, and a high specific surface area. Recently, graphene-based nanomaterials that are, in the literature, variously called graphene, carbon nanoflakes, carbon nanoflowers, carbon nanohorns, carbon nanowalls, or graphene nanosheets (GNSs), have attracted scientific attention due to their unique dimensions, structure, and electronic properties, which make them promising candidates for many applications. Such structures shall be referred to in this document as graphene sheets or graphene nanosheets. Possible applications for graphene nanosheets include use in electron field emitters, electrochemical capacitors, electrode material for capacitive deionisation, anode materials for lithium-ion batteries, catalyst supports, biosensors, electrodes for fuel cells, photocatalytic applications, transparent conducting films and nanocontactors. Other potential applications may include or involve corrosion prevention, conducting inks, lubricants, more efficient solar cells, novel antibiotics, and filler in new ultra high performance polymer-, ceramic- and metal-based composites. In addition to these, graphene/semiconductor nanocomposites are promising new class of catalysts for the photodegradation of dye pollutants. Graphene also provides new opportunities to advance water desalination technologies, and challenges the current existing adsorbents employed for the removal of low concentrated contaminants from aqueous solutions. Also, graphene nanosheets can be used as templates for fabrication of other nanostructured materials.
Graphene sheets were produced for the first time in small amounts by an “up to bottom” approach of micromechanical cleavage of highly oriented pyrolytic graphite (HOPG). Later, relatively larger amounts of chemically modified graphene sheets were produced by a number of approaches, all of which made use of HOPG as the starting material and involved labour-intensive preparations. More recently there has been a focus on the preparation of graphene sheets using similar methods to those employed in the production of carbon nanotubes. For example, graphene sheets have been synthesized by chemical vapour deposition (CVD) techniques on a substrate as vertically aligned carbon sheets having an average thickness of several nanometers. Graphene sheets have also been synthesized by plasma enhanced CVD (PECVD), hot-wire CVD, dc-plasma enhanced CVD (dc-PECVD), radiofrequency (rf)-PECVD, inductively coupled PECVD, inductively coupled rf-PECVD, glow discharge PECVD, microwave discharge CVD, electron beam excited PECVD, and also by pyrolysis-based methods.
CVD-based synthesis methods for graphene sheets suffer from low production rates, which can be as low as 32 nm min−1. If a surface area of 1 m2 is used for the CVD process, the rate of graphene production is typically less than 1 g per day. Moreover, these methods require complex equipment. As an alternative to CVD processes, the exfoliation of graphite into carbon nanomaterials by room temperature ionic liquids has been subject of a number of studies. It has been shown that the electrolysis of room temperature ionic liquids with graphite electrodes may lead to some erosion or exfoliation of the graphite electrode material into carbon nanostructures including graphene sheets. However, the rate of the synthesis of graphene in room temperature ionic liquids is low. Moreover, room temperature ionic liquids are mostly toxic, non-biodegradable and too expensive.
At present, no graphene sheet production process exists that is capable of supplying large amounts of graphene sheets or graphene-based materials. Thus, the development of applications and materials using graphene is difficult.